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I was watching my Salmon-tailed Catfish this morning and noticed his right fin looks red/bloodied, and it looks like theres a tiny lump there too.

A crude diagram:



Is there anything I can do for him? Or is this just something that time will heal? (might explain why my catfish likes to sit on his butt all day if it hurts him to swim about)...

For the record, I only just got him 2 days ago from the local pet store/aquarium place...

That sounds exactly like the sympoms one of my clarias had. I dont know if my treatment or nature cured the infection though. You might want to try melafix to begin with. its pretty good with minor infections. But I would be probably just keep it under observation for a while. if it gets worse, then treat it.
However, in the end I resorted to more drastic measures to cure my fish and as i said before im not sure which worked best but the infection did disapear.
Silurus said that it might be an Aeromonas infection, and after a couple of weeks with a steadily worsening condition and the onset of minor finrot I decided to try a salt bath, then lastly out of desperation I tried applying neat tee tree oil to the wound with a cotton wool bud (Q tip) I think it was this that cured the little bugger but as I said before its hard to say.
Its symptoms wore a blodied fin, a small reddish lump on the fin which then also appeared on the side of its head.
I beleive that this was probably due to an ammonia spike in my tank combined with a high pH which coincided with an injury which I witnessed with my own eyes! The stoopid little so and so crashed into some driftwood in a feeding frenzy and blood bloomed from its fin/ gill area.
I hope your fish doesnt have the same thing mine had as it was a very stubborn infection, but curable.
In short, keep a close eye on him and the water parameters, do small regular waterchanges, and be sure not to overfeed.
Im sorry if that wasnt much help, but i think i have been relatively lucky, figers crossed, touch wood etc etc.
jason